Optical systems may be used, for example, in a telescope or in field glasses. By way of example, optical systems in the form of field glasses are known, which have two housings in the form of two tubes. A first imaging unit having a first optical axis is arranged in a first tube. A second imaging unit having a second optical axis is arranged in a second tube. Moreover, the prior art has disclosed field glasses which have a first housing in the form of a first tube with a first optical axis and a second housing in the form of a second tube with a second optical axis. The first housing is connected to the second housing by way of a folding bridge, with the folding bridge having a first hinge part arranged at the first housing and the folding bridge having a second hinge part arranged at the second housing. The folding bridge has a folding axis. If the two housings pivoted relative to one another about the folding axis, there is a change in the distance between the two housings.
The image captured by an observer through the telescope or the field glasses is often perceived to be shaking because trembling movements or rotational movements of the hands of the user, and also movements underfoot, in turn cause movements of the optical system. In order to avoid this, it is known to stabilize images in an optical system. Known solutions use stabilizing apparatuses for stabilizing the image by means of a mechanical apparatus and/or an electronic apparatus.
DE 23 53 101 C3 has disclosed an optical system in the form of a telescope, which has an objective, an image stabilizing unit in the form of a prism erecting system and an eyepiece. The prism erecting system is mounted in Cardan-joint fashion in a housing of the telescope. This is understood to mean that the prism erecting system is arranged in the housing of the telescope such that the prism erecting system is mounted such that it can rotate about two axes arranged at right angles to one another. For the rotatable mounting, use is generally made of a device which is referred to as a Cardan-type mount. A hinge point of the erecting system, mounted in a Cardan-joint fashion in the housing, is arranged centrally between an image-side main plane of the objective and an object-side main plane of the eyepiece. The prism erecting system, mounted in a Cardan-joint fashion, is not moved by occurring rotational movements as a result of its inertia. It therefore remains fixed in space. This is how an image deterioration which occurs as a result of the movement of the housing is compensated for.
DE 39 33 255 C2 discloses binocular field glasses with an image stabilizing unit having a prism erecting system. The prism erecting system has Porro prisms, which respectively have one tilt axis. The Porro prisms are designed such that they can pivot about their respective tilt axis. Motors are provided for pivoting the Porro prisms. The pivoting is brought about dependent on a trembling movement which causes a shaking of an observed image.
Furthermore, U.S. Pat. No. 6,414,793 B1 has disclosed further binocular field glasses with an image stabilizing unit. U.S. Pat. No. 7,460,154 B2 has disclosed a device for compensating vibrations using a coordinate transformation.
As mentioned above, drive units (actuators) move the image stabilizing unit or at least one optical element of the image stabilizing unit in some of the known optical systems. These drive units are controlled by way of actuation signals, which are provided by a control unit or by a plurality of control units. As a result of the inertia of the mass of the image stabilizing unit or of the optical element of the image stabilizing unit, there is, in this case, a time delay between the initiation of the movement by the actuation signal in the drive units and the implementation of the actual movement of the image stabilizing unit or of the optical element of this image stabilizing unit. Consequently, the image stabilization in relation to, for example, an occurring trembling movement, is implemented with some time delay. This can have an influence on the quality of the image stabilization, as will be explained below.
In a simplified approach, the trembling movement can be considered to be a composition of numerous sinusoidal vibrations with different frequencies. If now only a single sinusoidal vibration with a specific frequency from the spectrum of numerous frequencies is considered, the above-described time delay leads to a shift in the phase between the phase of the actuation signal and the phase of the movement of the image stabilizing unit or of the optical element of the image stabilizing unit. Expressed differently, the time delay also leads to a shift in the phase between the trembling movement to be stabilized and the movement of the image stabilizing unit or of the optical element of the image stabilizing unit. This then also has an influence on a movement of the image of an object in the optical system, e.g. field glasses.
The shift in the phase increases with increasing frequency of the actuation signal. In the case of a very high frequency of the actuation signal, the shift in the phase is so high that the movement of the image stabilizing unit or of the optical element of the image stabilizing unit emerges in such a shifted manner that the movement of the image stabilizing unit or of the optical element of the image stabilizing unit does not suffice for compensating the trembling movement. The trembling movement and the movement of the image due to the image stabilization are clearly perceived.
What was mentioned above can be elucidated on the basis of FIGS. 8 and 9. FIGS. 8 and 9 plot the amplitudes of the vibrations of the movement of the image stabilizing unit or of the optical element of the image stabilizing unit, of the actuation signal and the visible difference emerging in the optical system from these vibrations over time. In the case of a delay in the onset of the movement of the image stabilizing unit or of the optical element of the image stabilizing unit of 5% of the period duration of the actuation signal, a movement of the image in the optical system of approximately 30% of the original trembling movement is generated (FIG. 8). In the case of a delay in the onset of the movement of the image stabilizing unit or of the optical element of the image stabilizing unit of 12.5% (corresponding to 45°) of the period duration of the actuation signal, the emerging visible difference is just as large as the actuation signal. Accordingly, the image of the object (i.e. the image) will move with an amplitude during the image stabilization that corresponds to the amplitude of the trembling movement. This is clearly visible to an observer. Then, this can no longer be referred to as image stabilization.
A further disadvantage of the aforementioned inertia is that the amplitude of the movement of the image stabilizing unit or of the optical element of the image stabilizing unit becomes smaller with increasing frequency. Expressed differently, a delayed movement of the image stabilizing unit or of the optical element of the image stabilizing unit with movements that are too small follows a quick change in the actuation signal.
Optical units which implement monitoring of the position of the image stabilizing unit or of the optical element of the image stabilizing unit and the closed-loop control thereof following an actuation signal are known for the purposes of avoiding the aforementioned problems. To this end, use is made of a PID control. The amplitudes of the movement of the image stabilizing unit or of the optical element of the image stabilizing unit are adapted by means of the PID control. Furthermore, it is possible to counteract the phase shift. However, the PID control should be able to control quickly. In order to be able to react to deviations from an intended value of an actuation signal, an excessively strong drive unit is used as a result of the above-described inertia problem. However, a drive unit with such a design requires much energy and installation space. Furthermore, it was found oscillatory phenomena occur in the case of the fast closed-loop control by means of the PID control which, firstly, reduce the quality of the image stabilization and, secondly, likewise increase the energy consumption.
Accordingly it is desirable to provide an optical system and a method for operating the optical system, in which the drive unit can be designed in such a way that it works in an energy efficient manner.